This invention relates generally to a multi-winding electric power supply system wherein separate windings of the supply can be alternatively connected in either series or parallel circuit relationship, and it relates more particularly to power supplies of this kind that are useful in electrical propulsion systems for traction vehicles.
Propulsion systems for traction vehicles such as locomotives commonly use a diesel engine prime mover to drive electric generating means for supplying energy to a plurality of pairs of direct current (d-c) traction motors. The generating means typically comprises a 3-phase traction alternator whose alternating voltage output is rectified and applied to relatively positive and negative d-c buses between which the respective pairs of motors are connected in parallel. The output power of the alternator is regulated or varied by suitably controlling the strength of its field excitation and the rotational speed of the engine. For maximum efficiency the controls of the propulsion system are suitably designed to work the engine on its full horsepower curve throughout a wide speed range of the locomotive.
In order to accelerate a locomotive from rest, the alternator must supply maximum current to the traction motor so that they can provide high tractive force or effort, but at low speed the output voltage of the alternator can be relatively low because the counter emf of each motor is a function of locomotive speed. When relatively high speed operation of the locomotive is desired, the alternator must apply maximum voltage to the traction motors to overcome their high counter emf, but the alternator output current can now be relatively low because the motors draw less current at high speed than at low speed. To accommodate both of these extremes without reducing the useful horsepower of the engine and without requiring an unreasonably large or expensive alternator, it has heretofore been common practice to provide speed responsive means for transitioning between parallel and series the circuits that interconnect the two motors forming each pair of traction motors in the propulsion system. At low speeds, when high current but low voltage is required, the motors in each pair are interconnected in series with one another, whereas at high speeds, when high voltage but low current is required, all of the motors are configured in a parallel mode. The change of modes is accomplished by means of suitable contactors in the motor circuits, which contactors are actuated automatically in response to the sensed speed of the locomotive traversing a predetermined critical speed between high and low speed ranges.
When an accelerating locomotive attains the aforesaid critical speed that initiates a transition of each pair of traction motors from series mode to parallel mode, all of the motors are temporarily disconnected from the d-c bus (to avoid undesirable short circuits) before they are reconnected in parallel. However, before this switching sequence begins the alternator field excitation level is reduced so that the output voltage of the alternator will be very low or zero when the series contactors are opened. If the output voltage were not lowered in this manner, it would be near its maximum value at the transition speed of the locomotive with each pair of motors still connected in series, and the series contactors might flash over when opened. Once the voltage has been lowered and the series contactors have been safely opened, and after the parallel contactors are subsequently closed, excitation is restored to the proper level for increasing the alternator output voltage to a new value which is approximately one-half of its value just before the locomotive attains the transition speed. The time required to complete this prior art series-to-parallel motor transition sequence, including the time to restore power to its desired level, has typically been appreciably longer than ten seconds. This method is accompanied by undesired loss in acceleration and tractive effort.
In the prior art arrangement summarized above, when the paired motors are connected in the series mode, a problem can arise if one of the locomotive axle-wheel sets loses adhesion and begins to slip on the rails. The particular traction motor that is coupled to the slipping wheel set will accelerate faster than the other motors, and this condition, if uncorrected, could result in rail grinding, wheel spalling, and motor overspeed. During the wheel slip condition the affected motor experiences increasing counter emf and decreasing current. The second motor with which the affected motor is serially paired will accordingly suffer the same decrease in current, and its voltage will decrease by an amount equal to the increase in voltage of the slipping motor. As a result neither motor can use the energy intended for it, and since the system regulates constant horsepower, power will shift to the non-slipping wheel sets, increasing their tendency to slip.
If all of the motors could be connected in parallel during low speed, high tractive effort operation of the locomotive, the effect of a slipping wheel set on the others would be less severe. That is, if a wheel slip occurs, the acceleration above rail speed can no longer be as drastic as in the series configuration because the voltage of the affected motor is clamped to the bus of the other motors. Also the shift in power out of the slipping motor will be absorbed by more motors than before, reducing the tendency to precipitate slips of other wheel sets. The parallel configuration, then, has inherent advantages in the control of wheel slips, and the net useable adhesion can be materially improved.
In order to keep the traction motors permanently in parallel, it has been previously proposed to utilize a dual winding alternator having two sets of 3-phase armature windings that can be connected either in parallel (for low speed, high current operation) or in series (for high speed, high voltage operation). See German Pat. No. 2,254,937, U.S. reissue patent Re. No. 23,314, and U.S. Pat Nos. 3,984,750 and 4,009,431. All of these known prior art approaches have involved providing individual rectifying bridges or units for each set of alternator windings and selectively switching the d-c outputs of the respective rectifier units between series and parallel connections in response to the transition-initiating event.